Move Scene.to_hvplot internals to _scene_converters

[bornagain1981]

As requested in PR #2106, the to_hvplot and to_geoviews methods are moved under _scene_converters.py

• Test added
• Fully documented

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e20ea41) 95.39% compared to head (f11db54) 95.39%.
Report is 37 commits behind head on main.

Files	Patch %	Lines
satpy/_scene_converters.py	94.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2699      +/-   ##
==========================================
- Coverage   95.39%   95.39%   -0.01%     
==========================================
  Files         371      371              
  Lines       52704    52690      -14     
==========================================
- Hits        50277    50263      -14     
  Misses       2427     2427              

Flag	Coverage Δ
behaviourtests	4.16% <8.92%> (+<0.01%)	⬆️
unittests	96.00% <94.64%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 7247648596

• 53 of 56 (94.64%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.955%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/_scene_converters.py	47	50	94.0%

Totals
Change from base Build 7246279156:	0.0%
Covered Lines:	50389
Relevant Lines:	52513

---

💛 - Coveralls

[mraspaud]

LGTM

[mraspaud]

I'll let @djhoese merge as he ask for this refactoring.

[djhoese]

Looks good. Thanks for doing this. As mentioned on slack the readthedocs failure is unrelated to your changes. I think I'll merge this PR and make a separate PR for the fixes. Thanks again.